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Re: Weak axis bending of cold-formed steel members

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There is a case where LTB is possible with weak-axis bending...there was a blurb about it in the August 2004 Modern Steel Construction.  I just skimmed it a few nights ago...something about loading a weak axis member above the shear center can cause LT effects. It was in response to the April (?) quiz answer, and the fellow who pointed out the issue has also written a book on LT buckling.  I looked online but wasn't able to find it.  If someone else still has the August issue, maybe they can scan the useful details for you.



At 10:23 AM 9/6/2004 -0700, you wrote:

List,

I was wondering if anyone could point me in the right direction concerning lateral-torsional buckling and weak axis bending for light gage members.  My case is for weak axis bending of a hat section, but I was also wondering about the general case.  I have the three AISI 2001 manuals and I believe I understand the different strengths that need to be calculated, however, the specification manual section C3.1.2 is for I-, Z-, C- and other symmetric section flexural members and not for U- sections ; however there is a Supplementary Part V Section 2 which does include U-sections .  It further states that the provisions of this section do not apply to laterally un-braced compression flanges of otherwise laterally stable sections.  I can t find anything that specifically states that you do not need to check this limit state, but I vaguely remember from my steel design courses (maybe incorrectly) that this does not occur for weak axis bending in general. 

 

My questions are:

 

1)  Is the limit state for Lateral-Torsional Buckling limited to major axis bending as it is in the AISC LRFD Manual section F1.2?

 

2)  Is weak axis bending considered a laterally stable section and therefore you don t have lateral-torsional buckling?  For example, a C-section in weak axis bending would not need to use the procedures of C3.1.2 or C3.1.3 (One Flange through-fastened to Deck or Sheathing).

 

3)  The Supplementary Part V Section 2 concerning hat sections has exclusions for 1) roof panels and 2) for a system comprised of flexural members and panels.  Does the 2nd exclusion occur only if you have more than 1 hat and therefore you have a system?

 

Thanks,

Bob

 

Bob Riley, P.E.

Mesiti-Miller Engineering, Inc.
224 Walnut Avenue, Suite B
Santa Cruz, CA  95060
voice: 831.426.3186 x105

fax: 831.426.6607

www.m-me.com

 

 

 

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